Information on the efficiency and kinetics of protein assimilation in humans is scanty and moreover controversial. This is mainly due to the lack of a reliable and non-invasive measuring technique. The recent availability of stable isotope labelled protein allowed to study protein assimilation using tracer techniques. Applying these techniques, we showed that protein digestibility depends both on characteristics of the ingested meal and on the digestive and absorptive capacity of the upper gastrointestinal tract. The latter is significantly impaired in pancreatic disease but is also compromised by some drugs often used in clinical practice. We moreover confirmed that a substantial amount of even easily digestible dietary protein escapes assimilation in the small intestine.